Snap: support arm64
Snap: support arm64

Explicitly mark arm64 as supported build platform. Curiously, this
doesn't seem to be needed to build on arm64...

nawawi/poedit,alexhenrie/poedit,vslavik/poedit,alexhenrie/poedit,alexhenrie/poedit,alexhenrie/poedit,nawawi/poedit,nawawi/poedit,nawawi/poedit,vslavik/poedit,nawawi/poedit,vslavik/poedit,vslavik/poedit,alexhenrie/poedit,vslavik/poedit